A wide variety of electrochemical cells, or “batteries,” are known in the art. In general, batteries are devices that convert chemical energy into electrical energy, by means of an electrochemical oxidation-reduction reaction. Batteries are used in a wide variety of applications, particularly as a power source for devices that cannot practicably be powered by centralized power generation sources (e.g., by commercial power plants using utility transmission lines).
Batteries can generally be described as comprising three components: an anode that contains a material that is oxidized (yields electrons) during discharge of the battery (i.e., while it is providing power); a cathode that contains a material that is reduced (accepts electrons) during discharge of the battery; and an electrolyte that provides for transfer of ions between the cathode and anode. During discharge, the anode is the negative pole of the battery, and the cathode is the positive pole. Batteries can be more specifically characterized by the specific materials that make up each of these three components. Selection of these components can yield batteries having specific voltage and discharge characteristics that can be optimized for particular applications.
Batteries containing lithium and sodium afford many potential benefits, because these metals are light in weight, while possessing high standard potentials. For a variety of reasons, lithium batteries are, in particular commercially attractive because of their high energy density, higher cell voltages, and long shelf-life.
Lithium batteries typically comprise from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Among such batteries are those having metallic lithium anodes and metal chalcogenide (oxide) cathodes, typically referred to as “lithium metal” batteries. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous aprotic organic solvents. Other electrolytes are solid electrolytes (typically polymeric matrixes) that contain an ionic conductive medium (typically a lithium containing salt dissolved in organic solvents) in combination with a polymer that itself may be ionically conductive but electrically insulating.
A lithium battery that uses an “insertion anode” rather than lithium metal is typically referred to as a “lithium ion” battery. Insertion or “intercalation” electrodes contain materials having a lattice structure into which an ion can be inserted and subsequently extracted. Rather than chemically altering the intercalation material, the ions slightly expand the internal lattice lengths of the compound without extensive bond breakage or atomic reorganization. Insertion anodes contain, for example, lithium metal chakcogenide, lithium metal oxide, or carbon materials such as coke and graphite. These negative electrodes are used with lithium-containing insertion cathodes. In their initial condition, the cells are not charged, since the anode does not contain a source of cations. Thus, before use, such cells must be charged in order to transfer cations (lithium) to the anode from the cathode. During discharge the lithium is then transferred from the anode back to the cathode. During subsequent recharge, the lithium is again transferred back to the anode where it reinserts. This back-and-forth transport of lithium ions (Li+) between the anode and cathode during charge and discharge cycles has led to these cells being called “rocking chair” batteries.
A variety of materials have been suggested for use as cathode active materials in lithium batteries. Such materials include, for example, MoS2, MnO2, TiS2, NbSe3, LiCoO2, LiNiO2, LiMn2O4, V6O13, V2O5, SO2 CuCl2. Transition metal oxides such as those of the general formula LixMyOz, are among those materials preferred in such batteries having intercalation electrodes. Other materials include lithium transition metal phosphates, such as LiFePO4, and Li3V2(PO4)3. Such materials having structures similar to olivine or NASICON materials are among those known in the art.
Transition metal phosphate active materials are typically synthesized in a solid state reaction. Starting materials in particle form are mixed to produce an intimate mixture of particles. When heat is applied to effect reaction, the solid particles react with one another through a variety of surface reactions accompanied by diffusion of reactive materials into and out of the various particles in the mixture.
In general, such a cathode material must exhibit a high free energy of reaction with lithium, be able to intercalate a large quantity of lithium, maintain its lattice structure upon insertion and extraction of lithium, allow rapid diffusion of lithium, afford good electrical conductivity, not be significantly soluble in the electrolyte system of the battery, and be readily and economically produced. However, many of the cathode materials known in the art lack one or more of these characteristics. Moreover, the method by which such materials are made may also have an effect on one or more of these characteristics.